This invention relates to polyimides and, more particularly, to polyimides containing 4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl moieties.
Aromatic polyimides and polyamide-imides have found extensive use in industry as fibers, composites, molded parts and dielectrics due to their toughness, flexibility, mechanical strength and high thermal stability. In particular, polyimides are useful in the electronic industry due to their lower dielectric constant and high electrical resistivity. Such polymers have been used in both film and coating form as advanced materials for such uses as passivation and insulating coatings, interlevel dielectrics, die attach adhesives, flexible circuit substrates, and the like.
Polyamic acids, particularly in solution form, are extensively used to form coatings, such as passivation and insulating coatings. The polymers are generally made by dissolving the diamine in a compatible solvent and then adding the dianhydride to make a solution of polyamic acid. The resulting solution of polyamic acid is spread on a substrate, and the solvent evaporated to imidize fully the polyamic acid and make the coating. The polyamic acid, then, must be soluble in the solvent.
High molecular weight polymers generally provide increased thermal degradation stability and improved mechanical properties, such as tensile strength and elongation. For example, polyimides made from 4,4'-bis(4-aminophenoxy)biphenyl have the toughness, flexibility and thermal degradation stability necessary to meet the rigorous processing and operating conditions required for certain uses. However, they do not exhibit the combination of high thermal degradation temperature, low dielectric constant, low moisture uptake, and low glass transition temperature (Tg) required for certain electronic applications.
Polyimides, in general, are used as coatings over a topographical (i.e., nonplanar) surface on a substrate. As the polymer solution begins to dry (or cure), the polymer flows to maintain a planar surface. However, the flow ceases as the solvent evaporates and as the solution becomes more viscous, but before the film is completely dry. Once the polymer ceases to flow, the film shrinks and conforms to the nonplanar substrate rather than forming a planar surface. In other words, planarization is limited by the shrinkage that occurs after the film can no longer flow quickly enough to keep up with the drying (and imidization) process.
The formation of a planar polymer coating surface is particularly important in electronic applications where two or more layers of coated substrate can be stacked, one on top of the other, and the substrate layers separated by the planarized polyimide. Ideally, planarization would yield a planar surface, regardless of the complexity of the underlying topography.
A high glass transition temperature generally is a desirable polymer property; however, in the electronic industry a very high glass transition temperature is believed to have an adverse effect on planarization. Thus, it is desirable to have a polyimide which approaches a glass transition temperature range that will satisfy the minimum thermal requirements necessary for electronic applications, but which will allow the polyimide to flow longer (or until a higher percent of solvent is removed) and shrink less to form a more planar surface.
DE 3,490,169 discloses polymers made from a 4,4'-bis(4-amino-3-trifluoromethylphenoxy)biphenyl isomer. In other words, the German reference is directed to a particular isomer wherein a trifluoromethyl (CF.sub.3) group is in a position that is ortho to the amine group. The reported Tg's of polyimides of this reference which are made from 4,4'-bis(4-amino-3-trifluoromethylphenoxy)biphenyl are very high, in excess of 400.degree. C. (see Example 1), which would make this isomer unsuitable for planarization in a multilayer application. Further, a bulky group which is ortho to the amine groups can lower the reactivity of the diamine making the diamine difficult to polymerize. This reference also discloses a generic phenoxybiphenyl formula containing CF.sub.3 groups attached to one or more aromatic rings; however, there is no specific disclosure of the isomer of the present invention or of any polyimide made therefrom.